You most likely didn't bundle your mod correctly. Providing more information or images from Unity would likely be helpful.

For masterbundles, it's important that you tag your folders (and only the necessary folders) correctly. Additionally, your file path in Unity should be 1:1 with your file path in-game.

Sorry that it took me a while to respond to you again! I was looking through some other errors & work. I wrote up some potential issues / fixes that I was going to send you, but since you've fixed the problem it seems a bit moot now.

I'm glad that you were able to fix the issue on your own though!

I'll leave a shorter version of my suggestions below. Although you don't need them anymore, you might still find them helpful. Or maybe someone else will a similar issue come across them:

What I was originally going to say:

I took a closer look at the images you posted. Here's my current thoughts on potential issues:

• Unity 2018.4.36f1 is very outdated. You'll need to either update Unity or adjust your MasterBundle.dat file.
• Make sure you've tagged your main folder with an asset label.
• Double-check that your MasterBundle.dat is correct.

You can use older Unity versions, but it's better to use the same version as the game. The current version is Unity 2020.3.34f1. If you'd like to use Unity 2018 instead, you need to add Asset_Bundle_Version 3 to your MasterBundle.dat file.

Make sure the "SurivalIsland MasterBundle" folder is tagged with your asset label!

The information in your MasterBundle.dat file should be identical to how they are in Unity.

• Asset_Bundle_Name should be identical to whatever your asset label is named in Unity. I can't tell what yours should be based on just the info you've provided, but if your asset label was "surival.masterbundle" then you'd use: Asset_Bundle_Name surival.masterbundle.
• Asset_Prefix should be Asset_Prefix Assets/SurivalIsland MasterBundle, based on the images provided.
• Asset_Bundle_Version is only necessary if you're using an older Unity version. It's better to just update to the same version as the game uses instead.